A series of three experiments was performed to determine the minimum contrast necessary for a grating pattern to engage in binocular rivalry with a dissimilar pattern presented to the other eye. It was found that a pattern at its own contrast threshold can suppress temporarily a contralateral pattern, regardless of the contrast of the latter. In a fourth experiment it was found that a near-threshold pattern is as effective, in terms of the duration of visibility, as one that is four times greater in contrast; only the average duration of suppression varied with pattern contrast. This last set of data extends the findings of others to near-threshold conditions and poses difficulties for models of binocular rivalry based on the notion of reciprocal inhibition.